This invention relates generally to airborne paper web dryers and, more particularly, to airborne paper web dryers of the non-impingement or underpressure type over which a web travels in a running plane supported by a gas flow.
The provision of blow boxes in paper manufacturing and refining machines for supporting a travelling paper web in a manner such that the web does not physically contact any of the elements of the machine, that is, where the web is supported by appropriately directed gas flow, for purposes of web cleaning, drying and stabilizing are known. In such apparatus, the blown gas is directed through various types of nozzle equipment onto one or both sides of the web, after which the gas is drawn into subsequent nozzle apparatus for reuse. Of course, such gas has been previously heated to effectuate drying of the web.
Thus, conventional blow box apparatus used in airborne web drying comprise a set of nozzles which direct a gas flow on the travelling web for supporting and drying the same. Such conventional apparatus can be divided into two groups, namely, over-pressure or impingement type nozzles and underpressure or vacuum type nozzles. Blow box apparatus of the overpressure type employ the so called air-cushion principle in which air jets are directed to impinge against the web to provide a static overpressure in the space between the blow box and the web. Blow boxes employing under-pressure include nozzles which direct gas flow in a direction substantially parallel to the web resulting in a air foil effect that attracts the web and stabilizes its run. The attracting force applied on the web in such cases is based on the well known principle whereby a gas flow field creates a static vacuum between the web and the supporting surface of the blow box. In both overpressure and underpressure nozzles, the so called Coanda phenomenon is often used in order to direct the air flow in a desired direction.
The use of conventional overpressure or impingement type nozzles has not been entirely satisfactory. More particularly, such overpressure blow boxes have nozzles which direct sharp air jets against the web. Although the air jet provides effective heat transfer in the localized area where the air jet impinges against the web, this fact results in an uneven heat transfer longitudinally along the web which may have a detrimental influence on the resulting quality of the web. Additionally, it is difficult to treat a web on one side only when using blow boxes of the overpressure type since the web tends to separate from the blow box apparatus due to the impingement of the air jets thereon.
Reference is made to U.S. Pat. Nos. 3,587,177 and 3,711,960 and Finnish patent No. 42522 and DE Announcement Publication No. 2,020,430, which relate to the present subject matter.
In particular, U.S. Pat. No. 3,587,177 discloses an underpressure nozzle wherein the nozzle slot opens on the entry side of the supporting surface of the blow box and extends to the curved flow guide surface attached to the front end of the supporting surface of the blow box so as to direct the flow to follow the curved guide surface due to the above mentioned Coanda phenomenon. Upon reaching the exit side of the curved guide surface, the gas flow is parallel with the web. A drawback of the blow box structure illustrated in this patent which is typical of conventional blow boxes of the underpressure type is that since the gas flow is directed along the supporting surface of the blow box, the thermal transfer coefficient between the gas flow and the web is relatively low. Furthermore, since the gas flow which was initially heated has tended to cool by virtue of its action in preceeding blow boxes, the temperature differential between the web and the drying gas is reduced resulting in a consequent reduction in the thermal transfer capacity which, as known, is proportional to the product of the temperature difference and the thermal transfer coefficient. Yet another problem with conventional underpressure type blow boxes is that the distance between the web and the supporting surface of the blow box is relatively small, approximating 2 to 3mm, which fact results in the danger of the web touching the support surface of the blow box with consequent web rupture and/or fouling of the nozzle surfaces.